Baths for the chemical deposition of metal, also known as electroless metal deposition baths to distinguish from galvanic baths, have enjoyed increasing use in the metallizing of normally electrically non-conductive materials, among other applications. In practice, in such methods a layer of metal is built up on the surface of the insulating material entirely by means of chemical deposition, or an electrically conductive layer of metal produced by chemical deposition is treated further to deposit additional metal by means of electrolytic techniques.
Chemical metallizing baths principally contain ions of the metal to be deposited, a complexing agent for such ions, a reducing agent for such ions and a pH-adjustor. In general, such baths also include stabilizers, as well as agents for improving the ductility, tensile strength, structure and other characteristics of the metal deposited.
By the oxidation of the reducing agent in specific areas, i.e., at catalytic nuclei on the surface of the article being treated, electrons necessary for the conversion of the metal ions into elemental metal are delivered into the bath. The oxidation, and hence the deposition of metal, is triggered by such catalytic nuclei, which are formed from precious metals and certain other metals or metal compounds. In general, solutions are employed for such baths in which the deposited metal also catalyzes the oxidation and thus the further deposition of metal. Such solutions are referred to as autocatalytic metallizing baths.
During the operation of the bath, the metal ions, reducing agent and other bath constituents are consumed. This results in a falling off in the rate of metal deposition and, eventually, to a complete cessation of metal deposition. It has become the practice to replenish such baths, either continuously or intermittently, by adding further amounts of the constituents being consumed. The replenishment is controlled by ordinary batch-wise chemical analysis, or by the use of automatic analyzing or proportioning devices.
During such replenishments, care must be taken to ensure that local conditions do not arise which result in bath instability or in the formation of additional catalytic nuclei which can cause the uncontrolled deposition of metal or the destruction of the bath itself. Moreover, when additions of chemicals are made it is difficult to avoid the introduction of foreign ions which interfere with the deposition process, or to do so under economically justifiable conditions.
Another significant disadvantage of prior art methods of operating chemical metallizing baths is that the further addition of consumable bath ingredients results in increases in the volume of the bath. This necessitates the removal of excess amounts of bath liquid, e.g., by skimming off the overflow, or by other suitable means, even though the excess is useful. It has been proposed that such increases in bath volume can be kept low by adding the consumable bath ingredients in the form of concentrated solutions. However, such methods have enjoyed only limited used because the replenishment of the metal ion to be deposited, usually in the form of a soluble metal salt, also makes it necessary to add more of the pH adjustor. Moreover, other salts form as by-products, and this leads to increases in the bath density.
For example, in the case of copper chemical deposition baths having a pH value in the alkaline range, either alkali metal sulfates or alkali metal chlorides are formed, dependng upon the particular copper salt used. In addition, in such baths copper formates also appear as by-products when formaldehyde is employed as the reducing agent. Because the activity of the bath and the quality of the metal being deposited are adversely affected by high bath densities, it is desirable to keep the density within a specified range. To this end, more water is added to dilute the bath, but this leads to further increases in the bath volume and a loss of useable bath liquid by overflow.
It is known that, for purposes of economy and environmental protection, the overflow of excess liquid from such baths can be treated to remove metals, e.g., nickel, copper, and the like, as well as complexing agents for such metals, and to remove or break down other bath constituents which are harmful to the environment. The devices which are suitable for achieving the foregoing also complicate the operation of chemical metallizing plants and tend to increase manufacturing costs, however.
It is a principal object of this invention to provide a process which permits the operation of chemical metallizing baths without the aforementioned shortcomings of prior art methods of operation.